Historically many types of trucks have used hydraulic pressure to power the towed trailer. For instance, the construction business has long used a type of dump trailer, commonly called an “end dump”, to haul gravel from one location and dump the gravel at the destination. The end dump trailer is typically pulled by a Class 8 on-highway tractor. Power to operate the hydraulic pump for the hydraulic fluid is generated by a power take off (“PTO”) on the tractor, and the cylinder used for dumping the load is commonly mounted on the trailer. The operating pressure of a telescopic cylinder on a typical end dump is 2100 PSI max, with 30 gallons per minute of nominal flow of the hydraulic fluid. This type of end dump requires a 3-way directional valve mounted on the tractor to raise and lower the telescopic cylinder. In the most common arrangement for connecting the cylinder on the trailer to the 3-way directional valve, a single hydraulic line attached integrally to the hydraulic oil pump is used.
In the past couple of decades, other hydraulically powered trailers have become common. For instance, a trailer known as a “walking floor trailer” has become common for hauling bulk commodities such as garbage, wood chips, sawdust and grain. A common walking floor trailer requires up to 2800 PSI hydraulic operating pressures at a nominal flow of 30 gallows per minute to move the floor with heavy loads. No directional valve is typically mounted on the tractor used for the walking floor trailer, as the operational valving is typically located on the trailer. Two lines, a high pressure line and a return line, are required to connect the PTO driven hydraulic fluid from the tractor to the trailer. The PTO, hydraulic pump, reservoir and associated piping or tubing mounted on the tractor to hydraulically power the trailer has become commonly called a “wet kit”.
When walking floor trailers started becoming popular, fleet owners wanted the flexibility to pull both end dump trailers and walking floor trailers. This gave the fleet owner more revenue options. Combination wet kits were assembled in-situ and mounted on the tractor. Such combination wet kits might include a 4-way directional valve mounted on the tractor, with 2800 PSI of hydraulic fluid pressure available on one port of the directional valve and with a work port relief mounted on another port of the directional valve to limit the working pressure for end dump trailer use.
Another type of hydraulically powered trailer has also become popular with construction companies. A trailer known as a “side dump” trailer includes a payload that dumps by rotating the box about a longitudinal axis to dump the load to the side of the trailer. Side dump trailers use about the same hydraulic flow rate (25-30 GPM) as end dump and walking floor trailers, but require an intermediate pressure of about 2500 PSI.
Hydraulic packages were further modified to include sufficient pressure relief valving and appropriate output ports, connected as add-ons to the existing systems. The cost of all the various valves and piping needed together with its piece-by-piece assembly on the tractor has become time consuming and expensive. Each package is essentially a custom job of ordering or stocking the various parts, connections and fittings, and then a custom plumbing job of assembling the combination wet kit piece by piece on the tractor. Additionally, the multiple fitting connections became potential leak points for hydraulic fluid during the years of duty and wear to which the hydraulic system would be subjected.
One solution to combo wet kits utilizes a single high pressure connection for all types of trailers. However, the telescopic cylinders on most end dump trailers are only rated at pressures up to 2100 PSI on a continuous basis with a maximum pressure rating of 2500 PSI. Exceeding rated continuous and maximum pressures has the potential to fatigue the cylinder retaining rings and cause the cylinder stages to separate, usually when fully extended. Such a catastrophic failure causes the cylinder pieces, the dump box and the load to come crashing down, with severe risk to life and limb of anyone in the vehicle or its vicinity. Better solutions are needed.